A major developmental challenge for children with Down Syndrome (DS) is generating comprehensible and accurate speech within the context of a dynamic, maturing system, which is mediated by input from immature, dynamic speech processing ability. It is perhaps not surprising that the development of speech often goes awry in DS, with speech disorder being a relatively high incidence condition that typically persists through adolescence into adulthood and is often a life-long disabling characteristic of DS. Because of this, it is important to develop more effective treatments that improve two key elements of speech disorders: Speech-comprehensibility and speech accuracy. In addition, because children with DS display considerable heterogeneity in abilities that affect differential response to speech treatment methods, it is important to examine the relationship between particular pretreatment skills as potential predictors or moderators of growth and differential response to treatment for speech-comprehensibility and speech accuracy. The purpose of this project is to examine the relationship between pretreatment levels of: Cognitive ability, speech-motor ability, degree of speech accuracy in elicited and spontaneous contexts, degree of speech- comprehensibility in spontaneous contexts, EEG/ERP measures of auditory differentiation of phonemes and (a) growth in levels of speech-comprehensibility and speech accuracy within children with DS who experienced speech therapy and (b) differential response to two types of speech treatment designed to increase levels of speech accuracy and speech comprehensibility in these children. Sixty children with DS who all have severe speech disorders at pretreatment will be recruited in this project. At entry into the study, auditory differentiation of phonemes will be measured using PCA and ICA methods, as will levels of speech accuracy and speech comprehensibility. The children with DS will then be randomly assigned to one of two treatments designed to improve speech skills: Phonological Knowledge Recast (PKR) treatment or Articulatory Training (AT) treatment. These children will receive treatment in a 1:1 format for 60 minutes/session, 2/week for 6 months. Speech-comprehensibility and speech accuracy will be subsequently tested again using the spontaneous sampling methods: a) twice during the treatment phase, b) immediately after treatment, and c) at two follow-up periods. All EEG/ERP measures, standardized test measures, speech-motor measures, and measures of speech accuracy, and speech-comprehensibility will also be repeated at the last period. Multiple regression, multivariate permutation tests, and growth curve analyses will be used to test the research questions. Five years of funding are requested. Relevance of this research to public health: People with DS often have difficulty developing accurate and comprehensible speech, which in turn has detrimental impacts on language, educational and social development. This project is designed to develop more effective treatments for speech disorders in DS.